1. Field of the Invention
The present invention generally relates to a process for selectively depositing diamond films, more particularly to a process for selectively depositing diamond films with the gas source of a mixture of C.sub.x H.sub.y plus CO.sub.2 or C.sub.x H.sub.y O.sub.z plus CO.sub.2. The present process include two stages of diamond deposition. In the period between the first and second stage, the substrate is immersed in an aqueous solution of HF plus HNO.sub.3 to increase the selectivity of diamond growth.
2. Description of the Prior Art
Diamond films are very suitable for semiconductor applications because of their excellent electrical properties such as wide band gap, high breakdown voltage and high hole mobility. However, a suitable mask material for patterning diamond is difficult to find, since an etchant solution which can etch diamond will also destroy the mask. Forming a diamond pattern by a chemical etching method is almost impossible.
Therefore, in recent years, the selective growth of diamond has been studied by many workers. For example, Hirabayashi et al. disclose a process for selectively growing diamond, which involves roughening a silicon substrate with diamond powders, selectively etching the diamond nuclei on the undesired region of the substrate by Ar.sup.+ ion beam, and finally depositing diamond films (Appl. Phy. Lett. 53, 1815 (1988)).
However, the above process suffers from certain disadvantages, namely, the growth rate is slow, the crystallinity is poor and the diamond selectivity is not acceptable.
Therefore, it is an object of the present invention to solve the above-mentioned problems and to provide an improved process for selectively depositing diamond films. Depositing diamond films by the process of the present invention, results in a rapid growth rate and diamond films with good crystallinity and selectivity.
To achieve the above object, the process for selectively depositing diamond films of the present invention includes the steps of:
(a) forming an etch-resistant layer on the silicon substrate;
(b) removing part of the etch-resistant layer to form a predetermined pattern, thereby covering part of the substrate and exposing part of the substrate;
(c) forming diamond nucleation sites on the silicon substrate, thereby making the diamond nucleation density on the exposed part of the substrate substantially greater than the diamond nucleation density on the covered part of the substrate by several orders;
(d) removing the remainder of the etch-resistant layer;
(e) a first deposition stage: depositing diamond films on the silicon substrate with a gas mixture of hydrocarbon (C.sub.x H.sub.y) plus CO.sub.2 or a gas mixture of oxygen-containing hydrocarbon (C.sub.x H.sub.y O.sub.z) plus CO.sub.2 ;
(f) immersing the silicon substrate in a hydrofluoric acid and nitric acid-containing aqueous solution to etch the diamond nuclei on the etch-resistant-layer-covering part of the silicon substrate; and
(g) a second deposition stage: depositing a diamond film on the silicon substrate with a gas mixture of hydrocarbon (C.sub.x H.sub.y) plus CO.sub.2 or a gas mixture of oxygen-containing hydrocarbon (C.sub.x H.sub.y O.sub.z) plus CO.sub.2.
According to one aspect of the present invention, the gas source for depositing diamond is a mixture of C.sub.x H.sub.y (or C.sub.x H.sub.y O.sub.z) plus CO.sub.2, without the use of conventional used hydrogen. By using such a gas source, the diamond will have a faster growth rate and better crystallinity.
According to the second aspect of the present invention, the silicon substrate is immersed in a HF and HNO.sub.3 -containing aqueous solution in the period between the first and second stage of diamond deposition. By this procedure, the undesired region of the diamond nuclei will be etched, thus resulting in better diamond selectivity.